KNL1-Bubs and RZZ Provide Two Separable Pathways for Checkpoint Activation at Human Kinetochores

Interplay of kinetochores and catalysts drives rapid assembly of the mitotic checkpoint complex

The spindle assembly checkpoint (SAC) ensures mitotic exit occurs only after sister chromatid biorientation, but how this coordination is mechanistically achieved remains unclear. Kinetochores, the megadalton complexes linking chromosomes to spindle microtubules, contribute to SAC signaling. However, whether they act solely as docking platforms or actively promote the co-orientation of SAC catalysts such as MAD1:MAD2 and BUB1:BUB3 remains unresolved. Here, we reconstitute kinetochores and SAC signaling in vitro to address this question. We engineer recombinant kinetochore particles that recruit core SAC components and trigger checkpoint signaling upon Rapamycin induction, and test their function using a panel of targeted mutants. At approximately physiological concentrations of SAC proteins, kinetochores are essential for efficient mitotic checkpoint complex (MCC) assembly, the key effector of SAC signaling. Our results suggest that kinetochores serve not only as structural hubs but also as catalytic platforms that concentrate and spatially organize SAC components to accelerate MCC formation and ensure timely checkpoint activation.

Distinct checkpoint and homolog biorientation pathways regulate meiosis I in Drosophila oocytes

Mitosis and meiosis have two mechanisms for regulating the accuracy of chromosome segregation: error correction and the spindle assembly checkpoint (SAC). We have investigated the function of several checkpoint proteins in meiosis I of Drosophila oocytes. Increased localization of several SAC proteins was found upon depolymerization of microtubules by colchicine. However, unattached kinetochores or errors in biorientation of homologous chromosomes do not induce increased SAC protein localization. Furthermore, the metaphase I arrest does not depend on SAC genes, suggesting the APC is inhibited even if the SAC is not functional. Two SAC proteins, ROD of the ROD-ZW10-Zwilch (RZZ) complex and MPS1, are also required for the biorientation of homologous chromosomes during meiosis I, suggesting an error correction function. Both proteins aid in preventing or correcting erroneous attachments and depend on SPC105R for localization to the kinetochore. We have defined a region of SPC105R, amino acids 123-473, that is required for ROD localization and biorientation of homologous chromosomes at meiosis I. Surprisingly, ROD removal from kinetochores and movement towards spindle poles, termed "streaming," is independent of the dynein adaptor Spindly and is not linked to the stabilization of end-on attachments. Instead, meiotic RZZ streaming appears to depend on cell cycle stage and may be regulated independently of kinetochore attachment or biorientation status. We also show that Spindly is required for biorientation at meiosis I, and surprisingly, the direction of RZZ streaming.

ZW10: an emerging orchestrator of organelle dynamics during the cell division cycle

Zeste white 10 (ZW10) was first identified as a centromere/kinetochore protein encoded by the ZW10 gene in Drosophila. ZW10 guides the spindle assembly checkpoint signaling during mitotic chromosome segregation in metazoans. Recent studies have shown that ZW10 is also involved in membrane-bound organelle interactions during interphase and plays a vital role in membrane transport between the endoplasmic reticulum and Golgi apparatus. Despite these findings, the precise molecular mechanisms by which ZW10 regulates interactions between membrane-bound organelles in interphase and the assembly of membraneless organelle kinetochore in mitosis remain elusive. Here, we highlight how ZW10 forms context-dependent protein complexes during the cell cycle. These complexes are essential for mediating membrane trafficking in interphase and ensuring the accurate segregation of chromosomes in mitosis.

Cyclin A/Cdk1 promotes chromosome alignment and timely mitotic progression

To ensure genomic fidelity, a series of spatially and temporally coordinated events is executed during prometaphase of mitosis, including bipolar spindle formation, chromosome attachment to spindle microtubules at kinetochores, the correction of erroneous kinetochore-microtubule (k-MT) attachments, and chromosome congression to the spindle equator. Cyclin A/Cdk1 kinase plays a key role in destabilizing k-MT attachments during prometaphase to promote correction of erroneous k-MT attachments. However, it is unknown whether Cyclin A/Cdk1 kinase regulates other events during prometaphase. Here, we investigate additional roles of Cyclin A/Cdk1 in prometaphase by using an siRNA knockdown strategy to deplete endogenous Cyclin A from human cells. We find that depleting Cyclin A significantly extends mitotic duration, specifically prometaphase, because chromosome alignment is delayed. Unaligned chromosomes display erroneous monotelic, syntelic, or lateral k-MT attachments suggesting that bioriented k-MT attachment formation is delayed in the absence of Cyclin A. Mechanistically, chromosome alignment is likely impaired because the localization of the kinetochore proteins BUB1 kinase, KNL1, and MPS1 kinase are reduced in Cyclin A-depleted cells. Moreover, we find that Cyclin A promotes BUB1 kinetochore localization independently of its role in destabilizing k-MT attachments. Thus, Cyclin A/Cdk1 facilitates chromosome alignment during prometaphase to support timely mitotic progression.

Distinct checkpoint and homolog biorientation pathways regulate meiosis I in Drosophila oocytes

Mitosis and meiosis have two mechanisms for regulating the accuracy of chromosome segregation: error correction and the spindle assembly checkpoint (SAC). We have investigated the function of several checkpoint proteins in meiosis I of Drosophila oocytes. Evidence of a SAC response by several of these proteins is found upon depolymerization of microtubules by colchicine. However, unattached kinetochores or errors in biorientation of homologous chromosomes does not induce a SAC response. Furthermore, the metaphase I arrest does not depend on SAC genes, suggesting the APC is inhibited even if the SAC is silenced. Two SAC proteins, ROD of the ROD-ZW10-Zwilch (RZZ) complex and MPS1, are also required for the biorientation of homologous chromosomes during meiosis I, suggesting an error correction function. Both proteins aid in preventing or correcting erroneous attachments and depend on SPC105R for localization to the kinetochore. We have defined a region of SPC105R, amino acids 123-473, that is required for ROD localization and biorientation of homologous chromosomes at meiosis I. Surprisingly, ROD removal, or "streaming", is independent of the dynein adaptor Spindly and is not linked to the stabilization of end-on attachments. Instead, meiotic RZZ streaming appears to depend on cell cycle stage and may be regulated independently of kinetochore attachment or biorientation status. We also show that dynein adaptor Spindly is also required for biorientation at meiosis I, and surprisingly, the direction of RZZ streaming.

The dynamic role of nucleoprotein SHCBP1 in the cancer cell cycle and its potential as a synergistic target for DNA-damaging agents in cancer therapy

BACKGROUND

Malignant tumours seriously threaten human life and health, and effective treatments for cancer are still being explored. The ability of SHC SH2 domain-binding protein 1 (SHCBP1) to induce cell cycle disturbance and inhibit tumour growth has been increasingly studied, but its dynamic role in the tumour cell cycle and corresponding effects leading to mitotic catastrophe and DNA damage have rarely been studied.

RESULTS

In this paper, we found that the nucleoprotein SHCBP1 exhibits dynamic spatiotemporal expression during the tumour cell cycle, and SHCBP1 knockdown slowed cell cycle progression by inducing spindle disorder, as reflected by premature mitotic entry and multipolar spindle formation. This dysfunction was caused by G2/M checkpoint impairment mediated by downregulated WEE1 kinase and NEK7 (a member of the mammalian NIMA-related kinase family) expression and upregulated centromere/kinetochore protein Zeste White 10 (ZW10) expression. Moreover, both in vivo and in vitro experiments confirmed the significant inhibitory effects of SHCBP1 knockdown on tumour growth. Based on these findings, SHCBP1 knockdown in combination with low-dose DNA-damaging agents had synergistic tumouricidal effects on tumour cells. In response to this treatment, tumour cells were forced into the mitotic phase with considerable unrepaired DNA lesions, inducing mitotic catastrophe. These synergistic effects were attributed not only to the abrogation of the G2/M checkpoint and disrupted spindle function but also to the impairment of the DNA damage repair system, as demonstrated by mass spectrometry-based proteomic and western blotting analyses. Consistently, patients with low SHCBP1 expression in tumour tissue were more sensitive to radiotherapy. However, SHCBP1 knockdown combined with tubulin-toxic drugs weakened the killing effect of the drugs on tumour cells, which may guide the choice of chemotherapeutic agents in clinical practice.

CONCLUSION

In summary, we elucidated the role of the nucleoprotein SHCBP1 in tumour cell cycle progression and described a novel mechanism by which SHCBP1 regulates tumour progression and through which targeting SHCBP1 increases sensitivity to DNA-damaging agent therapy, indicating its potential as a cancer treatment.

A coadapted KNL1 and spindle assembly checkpoint axis orchestrates precise mitosis in Arabidopsis

The kinetochore scaffold 1 (KNL1) protein recruits spindle assembly checkpoint (SAC) proteins to ensure accurate chromosome segregation during mitosis. Despite such a conserved function among eukaryotic organisms, its molecular architectures have rapidly evolved so that the functional mode of plant KNL1 is largely unknown. To understand how SAC signaling is regulated at kinetochores, we characterized the function of the KNL1 gene in Arabidopsis thaliana. The KNL1 protein was detected at kinetochores throughout the mitotic cell cycle, and null knl1 mutants were viable and fertile but exhibited severe vegetative and reproductive defects. The mutant cells showed serious impairments of chromosome congression and segregation, that resulted in the formation of micronuclei. In the absence of KNL1, core SAC proteins were no longer detected at the kinetochores, and the SAC was not activated by unattached or misaligned chromosomes. Arabidopsis KNL1 interacted with SAC essential proteins BUB3.3 and BMF3 through specific regions that were not found in known KNL1 proteins of other species, and recruited them independently to kinetochores. Furthermore, we demonstrated that upon ectopic expression, the KNL1 homolog from the dicot tomato was able to functionally substitute KNL1 in A. thaliana, while others from the monocot rice or moss associated with kinetochores but were not functional, as reflected by sequence variations of the kinetochore proteins in different plant lineages. Our results brought insights into understanding the rapid evolution and lineage-specific connection between KNL1 and the SAC signaling molecules.

Cyclin A/Cdk1 promotes chromosome alignment and timely mitotic progression

To ensure genomic fidelity a series of spatially and temporally coordinated events are executed during prometaphase of mitosis, including bipolar spindle formation, chromosome attachment to spindle microtubules at kinetochores, the correction of erroneous kinetochore-microtubule (k-MT) attachments, and chromosome congression to the spindle equator. Cyclin A/Cdk1 kinase plays a key role in destabilizing k-MT attachments during prometaphase to promote correction of erroneous k-MT attachments. However, it is unknown if Cyclin A/Cdk1 kinase regulates other events during prometaphase. Here, we investigate additional roles of Cyclin A/Cdk1 in prometaphase by using an siRNA knockdown strategy to deplete endogenous Cyclin A from human cells. We find that depleting Cyclin A significantly extends mitotic duration, specifically prometaphase, because chromosome alignment is delayed. Unaligned chromosomes display erroneous monotelic, syntelic, or lateral k-MT attachments suggesting that bioriented k-MT attachment formation is delayed in the absence of Cyclin A. Mechanistically, chromosome alignment is likely impaired because the localization of the kinetochore proteins BUB1 kinase, KNL1, and MPS1 kinase are reduced in Cyclin A-depleted cells. Moreover, we find that Cyclin A promotes BUB1 kinetochore localization independently of its role in destabilizing k-MT attachments. Thus, Cyclin A/Cdk1 facilitates chromosome alignment during prometaphase to support timely mitotic progression.

RZZ-Spindly and CENP-E form an integrated platform to recruit dynein to the kinetochore corona

Chromosome biorientation on the mitotic spindle is prerequisite to errorless genome inheritance. CENP-E (kinesin-7) and dynein-dynactin (DD), microtubule motors with opposite polarity, promote biorientation from the kinetochore corona, a polymeric structure whose assembly requires MPS1 kinase. The corona's building block consists of ROD, Zwilch, ZW10, and the DD adaptor Spindly (RZZS). How CENP-E and DD are scaffolded and mutually coordinated in the corona remains unclear. Here, we show that when corona assembly is prevented through MPS1 inhibition, CENP-E is absolutely required to retain RZZS at kinetochores. An RZZS phosphomimetic mutant bypasses this requirement, demonstrating the existence of a second receptor for polymeric RZZS. With active MPS1, CENP-E is dispensable for corona expansion, but strictly required for physiological kinetochore accumulation of DD. Thus, we identify the corona as an integrated scaffold where CENP-E kinesin controls DD kinetochore loading for coordinated bidirectional transport of chromosome cargo.

Signaling protein abundance modulates the strength of the spindle assembly checkpoint

During mitosis, unattached kinetochores in a dividing cell signal to the spindle assembly checkpoint (SAC) to delay anaphase onset and prevent chromosome missegregation.1,2,3,4 The signaling activity of these kinetochores and the likelihood of chromosome missegregation depend on the amount of SAC signaling proteins each kinetochore recruits.5,6,7,8 Therefore, factors that control SAC protein recruitment must be thoroughly understood. Phosphoregulation of kinetochore and SAC signaling proteins due to the concerted action of many kinases and phosphatases is a significant determinant of the SAC protein recruitment to signaling kinetochores.9 Whether the abundance of SAC proteins also influences the recruitment and signaling activity of human kinetochores has not been studied.8,10 Here, we reveal that the low cellular abundance of the SAC signaling protein Bub1 limits its own recruitment and that of BubR1 and restricts the SAC signaling activity of the kinetochore. Conversely, Bub1 overexpression results in higher recruitment of SAC proteins, producing longer delays in anaphase onset. We also find that the number of SAC proteins recruited by a signaling kinetochore is inversely correlated with the total number of signaling kinetochores in the cell. This correlation likely arises from the competition among the signaling kinetochores to recruit from a limited pool of signaling proteins, including Bub1. The inverse correlation may allow the dividing cell to prevent a large number of signaling kinetochores in early prophase from generating an overly large signal while enabling the last unaligned kinetochore in late prometaphase to signal at the maximum strength.

DNA damage and repair: underlying mechanisms leading to microcephaly

DNA-damaging agents and endogenous DNA damage constantly harm genome integrity. Under genotoxic stress conditions, the DNA damage response (DDR) machinery is crucial in repairing lesions and preventing mutations in the basic structure of the DNA. Different repair pathways are implicated in the resolution of such lesions. For instance, the non-homologous DNA end joining and homologous recombination pathways are central cellular mechanisms by which eukaryotic cells maintain genome integrity. However, defects in these pathways are often associated with neurological disorders, indicating the pivotal role of DDR in normal brain development. Moreover, the brain is the most sensitive organ affected by DNA-damaging agents compared to other tissues during the prenatal period. The accumulation of lesions is believed to induce cell death, reduce proliferation and premature differentiation of neural stem and progenitor cells, and reduce brain size (microcephaly). Microcephaly is mainly caused by genetic mutations, especially genes encoding proteins involved in centrosomes and DNA repair pathways. However, it can also be induced by exposure to ionizing radiation and intrauterine infections such as the Zika virus. This review explains mammalian cortical development and the major DNA repair pathways that may lead to microcephaly when impaired. Next, we discuss the mechanisms and possible exposures leading to DNA damage and p53 hyperactivation culminating in microcephaly.

The role of kinetochore dynein in checkpoint silencing is restricted to disassembly of the corona

During mitosis, kinetochore-microtubule attachments are monitored by a molecular surveillance system known as the spindle assembly checkpoint. The prevailing model posits that dynein evicts checkpoint proteins (e.g., Mad1, Mad2) from stably attached kinetochores by transporting them away from kinetochores, thus contributing to checkpoint silencing. However, the mechanism by which dynein performs this function, and its precise role in checkpoint silencing remain unresolved. Here, we find that dynein's role in checkpoint silencing is restricted to evicting checkpoint effectors from the fibrous corona, and not the outer kinetochore. Dynein evicts these molecules from the corona in a manner that does not require stable, end-on microtubule attachments. Thus, by disassembling the corona through indiscriminate microtubule encounters, dynein primes the checkpoint signaling apparatus so it can respond to stable end-on microtubule attachments and permit cells to progress through mitosis. Accordingly, we find that dynein function in checkpoint silencing becomes largely dispensable in cells in which checkpoint effectors are excluded from the corona.

A mechanism that integrates microtubule motors of opposite polarity at the kinetochore corona

Chromosome biorientation on the mitotic spindle is prerequisite to errorless genome inheritance. CENP-E (kinesin 7) and Dynein-Dynactin (DD), microtubule motors with opposite polarity, promote biorientation from the kinetochore corona, a polymeric structure whose assembly requires MPS1 kinase. The corona's building block consists of ROD, Zwilch, ZW10, and the DD adaptor Spindly (RZZS). How CENP-E and DD are scaffolded and mutually coordinated in the corona remains unclear. Here, we report near-complete depletion of RZZS and DD from kinetochores after depletion of CENP-E and the outer kinetochore protein KNL1. With inhibited MPS1, CENP-E, which we show binds directly to RZZS, is required to retain kinetochore RZZS. An RZZS phosphomimetic mutant bypasses this requirement. With active MPS1, CENP-E is dispensable for corona expansion, but strictly required for physiological kinetochore accumulation of DD. Thus, we identify the corona as an integrated scaffold where CENP-E kinesin controls DD kinetochore loading for coordinated bidirectional transport of chromosome cargo.

Principles and dynamics of spindle assembly checkpoint signalling

The transmission of a complete set of chromosomes to daughter cells during cell division is vital for development and tissue homeostasis. The spindle assembly checkpoint (SAC) ensures correct segregation by informing the cell cycle machinery of potential errors in the interactions of chromosomes with spindle microtubules prior to anaphase. To do so, the SAC monitors microtubule engagement by specialized structures known as kinetochores and integrates local mechanical and chemical cues such that it can signal in a sensitive, responsive and robust manner. In this Review, we discuss how SAC proteins interact to allow production of the mitotic checkpoint complex (MCC) that halts anaphase progression by inhibiting the anaphase-promoting complex/cyclosome (APC/C). We highlight recent advances aimed at understanding the dynamic signalling properties of the SAC and how it interprets various naturally occurring intermediate attachment states. Further, we discuss SAC signalling in the context of the mammalian multisite kinetochore and address the impact of the fibrous corona. We also identify current challenges in understanding how the SAC ensures high-fidelity chromosome segregation.

Dynein at the kinetochore

The microtubule minus-end-directed motility of cytoplasmic dynein 1 (dynein), arguably the most complex and versatile cytoskeletal motor, is harnessed for diverse functions, such as long-range organelle transport in neuronal axons and spindle assembly in dividing cells. The versatility of dynein raises a number of intriguing questions, including how is dynein recruited to its diverse cargo, how is recruitment coupled to activation of the motor, how is motility regulated to meet different requirements for force production and how does dynein coordinate its activity with that of other microtubule-associated proteins (MAPs) present on the same cargo. Here, these questions will be discussed in the context of dynein at the kinetochore, the supramolecular protein structure that connects segregating chromosomes to spindle microtubules in dividing cells. As the first kinetochore-localized MAP described, dynein has intrigued cell biologists for more than three decades. The first part of this Review summarizes current knowledge about how kinetochore dynein contributes to efficient and accurate spindle assembly, and the second part describes the underlying molecular mechanisms and highlights emerging commonalities with dynein regulation at other subcellular sites.

The Loss-Function of KNL1 Causes Oligospermia and Asthenospermia in Mice by Affecting the Assembly and Separation of the Spindle through Flow Cytometry and Immunofluorescence

KNL1 (kinetochore scaffold 1) has attracted much attention as one of the assembly elements of the outer kinetochore, and the functions of its different domains have been gradually revealed, most of which are associated with cancers, but few links have been made between KNL1 and male fertility. Here, we first linked KNL1 to male reproductive health and the loss-function of KNL1 resulted in oligospermia and asthenospermia in mice (an 86.5% decrease in total sperm number and an 82.4% increase in static sperm number, respectively) through CASA (computer-aided sperm analysis). Moreover, we introduced an ingenious method to pinpoint the abnormal stage in the spermatogenic cycle using flow cytometry combined with immunofluorescence. Results showed that 49.5% haploid sperm was reduced and 53.2% diploid sperm was increased after the function of KNL1 was lost. Spermatocytes arrest was identified at the meiotic prophase I of spermatogenesis, which was induced by the abnormal assembly and separation of the spindle. In conclusion, we established an association between KNL1 and male fertility, providing a guide for future genetic counseling regarding oligospermia and asthenospermia, and a powerful method for further exploring spermatogenic dysfunction by utilizing flow cytometry and immunofluorescence.

Kinetochore‐catalyzed MCC formation: A structural perspective

The spindle assembly checkpoint (SAC) is a cellular surveillance mechanism that functions to ensure accurate chromosome segregation during mitosis. Macromolecular complexes known as kinetochores, act as the interface of sister chromatid attachment to spindle microtubules. In response to unattached kinetochores, the SAC activates its effector, the mitotic checkpoint complex (MCC), which delays mitotic exit until all sister chromatid pairs have achieved successful attachment to the bipolar mitotic spindle. Formation of the MCC (composed of Mad2, BubR1, Bub3 and Cdc20) is regulated by an Mps1 kinase-dependent phosphorylation signaling cascade which assembles and repositions components of the MCC onto a catalytic scaffold. This scaffold functions to catalyze the conversion of the HORMA-domain protein Mad2 from an "inactive" open-state (O-Mad2) into an "active" closed-Mad2 (C-Mad2), and simultaneous Cdc20 binding. Here, our current understanding of the molecular mechanisms underlying the kinetic barrier to C-Mad2:Cdc20 formation will be reviewed. Recent progress in elucidating the precise molecular choreography orchestrated by the catalytic scaffold to rapidly assemble the MCC will be examined, and unresolved questions will be highlighted. Ultimately, understanding how the SAC rapidly activates the checkpoint not only provides insights into how cells maintain genomic integrity during mitosis, but also provides a paradigm for how cells can utilize molecular switches, including other HORMA domain-containing proteins, to make rapid changes to a cell's physiological state.

The first mitotic division of human embryos is highly error prone

Human beings are made of ~50 trillion cells which arise from serial mitotic divisions of a single cell - the fertilised egg. Remarkably, the early human embryo is often chromosomally abnormal, and many are mosaic, with the karyotype differing from one cell to another. Mosaicism presumably arises from chromosome segregation errors during the early mitotic divisions, although these events have never been visualised in living human embryos. Here, we establish live cell imaging of chromosome segregation using normally fertilised embryos from an egg-share-to-research programme, as well as embryos deselected during fertility treatment. We reveal that the first mitotic division has an extended prometaphase/metaphase and exhibits phenotypes that can cause nondisjunction. These included multipolar chromosome segregations and lagging chromosomes that lead to formation of micronuclei. Analysis of nuclear number and size provides evidence of equivalent phenotypes in 2-cell human embryos that gave rise to live births. Together this shows that errors in the first mitotic division can be tolerated in human embryos and uncovers cell biological events that contribute to preimplantation mosaicism.

Zombies Never Die: The Double Life Bub1 Lives in Mitosis

When eukaryotic cells enter mitosis, dispersed chromosomes move to the cell center along microtubules to form a metaphase plate which facilitates the accurate chromosome segregation. Meanwhile, kinetochores not stably attached by microtubules activate the spindle assembly checkpoint and generate a wait signal to delay the initiation of anaphase. These events are highly coordinated. Disruption of the coordination will cause severe problems like chromosome gain or loss. Bub1, a conserved serine/threonine kinase, plays important roles in mitosis. After extensive studies in the last three decades, the role of Bub1 on checkpoint has achieved a comprehensive understanding; its role on chromosome alignment also starts to emerge. In this review, we summarize the latest development of Bub1 on supporting the two mitotic events. The essentiality of Bub1 in higher eukaryotic cells is also discussed. At the end, some undissolved questions are raised for future study.

Structure of the RZZ complex and molecular basis of Spindly-driven corona assembly at human kinetochores

In metazoans, a ≈1 megadalton (MDa) multiprotein complex comprising the dynein-dynactin adaptor Spindly and the ROD-Zwilch-ZW10 (RZZ) complex is the building block of a fibrous biopolymer, the kinetochore fibrous corona. The corona assembles on mitotic kinetochores to promote microtubule capture and spindle assembly checkpoint (SAC) signaling. We report here a high-resolution cryo-EM structure that captures the essential features of the RZZ complex, including a farnesyl-binding site required for Spindly binding. Using a highly predictive in vitro assay, we demonstrate that the SAC kinase MPS1 is necessary and sufficient for corona assembly at supercritical concentrations of the RZZ-Spindly (RZZS) complex, and describe the molecular mechanism of phosphorylation-dependent filament nucleation. We identify several structural requirements for RZZS polymerization in rings and sheets. Finally, we identify determinants of kinetochore localization and corona assembly of Spindly. Our results describe a framework for the long-sought-for molecular basis of corona assembly on metazoan kinetochores.

Functional dissection of human mitotic genes using CRISPR-Cas9 tiling screens

In this Resource/Methodology, Herman et al. developed a method that leverages CRISPR–Cas9-induced mutations across protein-coding genes for the a priori identification of functional regions at the sequence level. As a test case, they applied this method to 48 human mitotic genes, revealing hundreds of regions required for cell proliferation, including domains that were experimentally characterized, ones that were predicted based on homology, and novel ones.

The identity of human protein-coding genes is well known, yet our in-depth knowledge of their molecular functions and domain architecture remains limited by shortcomings in homology-based predictions and experimental approaches focused on whole-gene depletion. To bridge this knowledge gap, we developed a method that leverages CRISPR–Cas9-induced mutations across protein-coding genes for the a priori identification of functional regions at the sequence level. As a test case, we applied this method to 48 human mitotic genes, revealing hundreds of regions required for cell proliferation, including domains that were experimentally characterized, ones that were predicted based on homology, and novel ones. We validated screen outcomes for 15 regions, including amino acids 387–402 of Mad1, which were previously uncharacterized but contribute to Mad1 kinetochore localization and chromosome segregation fidelity. Altogether, we demonstrate that CRISPR–Cas9-based tiling mutagenesis identifies key functional domains in protein-coding genes de novo, which elucidates separation of function mutants and allows functional annotation across the human proteome.

The Role of Mitotic Kinases and the RZZ Complex in Kinetochore-Microtubule Attachments: Doing the Right Link

During mitosis, the interaction of kinetochores (KTs) with microtubules (MTs) drives chromosome congression to the spindle equator and supports the segregation of sister chromatids. Faithful genome partition critically relies on the ability of chromosomes to establish and maintain proper amphitelic end-on attachments, a configuration in which sister KTs are connected to robust MT fibers emanating from opposite spindle poles. Because the capture of spindle MTs by KTs is error prone, cells use mechanisms that sense and correct inaccurate KT-MT interactions before committing to segregate sister chromatids in anaphase. If left unresolved, these errors can result in the unequal distribution of chromosomes and lead to aneuploidy, a hallmark of cancer. In this review, we provide an overview of the molecular strategies that monitor the formation and fine-tuning of KT-MT attachments. We describe the complex network of proteins that operates at the KT-MT interface and discuss how AURORA B and PLK1 coordinate several concurrent events so that the stability of KT-MT attachments is precisely modulated throughout mitotic progression. We also outline updated knowledge on how the RZZ complex is regulated to ensure the formation of end-on attachments and the fidelity of mitosis.

Spindle assembly checkpoint activation and silencing at kinetochores

The spindle assembly checkpoint (SAC) is a surveillance mechanism that promotes accurate chromosome segregation in mitosis. The checkpoint senses the attachment state of kinetochores, the proteinaceous structures that assemble onto chromosomes in mitosis in order to mediate their interaction with spindle microtubules. When unattached, kinetochores generate a diffusible inhibitor that blocks the activity of the anaphase-promoting complex/cyclosome (APC/C), an E3 ubiquitin ligase required for sister chromatid separation and exit from mitosis. Work from the past decade has greatly illuminated our understanding of the mechanisms by which the diffusible inhibitor is assembled and how it inhibits the APC/C. However, less is understood about how SAC proteins are recruited to kinetochores in the absence of microtubule attachment, how the kinetochore catalyzes formation of the diffusible inhibitor, and how attachments silence the SAC at the kinetochore. Here, we summarize current understanding of the mechanisms that activate and silence the SAC at kinetochores and highlight open questions for future investigation.

Molecular mechanism of Mad1 kinetochore targeting by phosphorylated Bub1

During metaphase, in response to improper kinetochore-microtubule attachments, the spindle assembly checkpoint (SAC) activates the mitotic checkpoint complex (MCC), an inhibitor of the anaphase-promoting complex/cyclosome (APC/C). This process is orchestrated by the kinase Mps1, which initiates the assembly of the MCC onto kinetochores through a sequential phosphorylation-dependent signalling cascade. The Mad1-Mad2 complex, which is required to catalyse MCC formation, is targeted to kinetochores through a direct interaction with the phosphorylated conserved domain 1 (CD1) of Bub1. Here, we present the crystal structure of the C-terminal domain of Mad1 (Mad1CTD ) bound to two phosphorylated Bub1CD1 peptides at 1.75 Å resolution. This interaction is mediated by phosphorylated Bub1 Thr461, which not only directly interacts with Arg617 of the Mad1 RLK (Arg-Leu-Lys) motif, but also directly acts as an N-terminal cap to the CD1 α-helix dipole. Surprisingly, only one Bub1CD1 peptide binds to the Mad1 homodimer in solution. We suggest that this stoichiometry is due to inherent asymmetry in the coiled-coil of Mad1CTD and has implications for how the Mad1-Bub1 complex at kinetochores promotes efficient MCC assembly.

Knl1 participates in spindle assembly checkpoint signaling in maize

The Knl1-Mis12-Ndc80 (KMN) network is an essential component of the kinetochore-microtubule attachment interface, which is required for genomic stability in eukaryotes. However, little is known about plant Knl1 proteins because of their complex evolutionary history. Here, we cloned the Knl1 homolog from maize (Zea mays) and confirmed it as a constitutive central kinetochore component. Functional assays demonstrated their conserved role in chromosomal congression and segregation during nuclear division, thus causing defective cell division during kernel development when Knl1 transcript was depleted. A 145 aa region in the middle of maize Knl1, that did not involve the MELT repeats, was associated with the interaction of spindle assembly checkpoint (SAC) components Bub1/Mad3 family proteins 1 and 2 (Bmf1/2) but not with the Bmf3 protein. They may form a helical conformation with a hydrophobic interface with the TPR domain of Bmf1/2, which is similar to that of vertebrates. However, this region detected in monocots shows extensive divergence in eudicots, suggesting that distinct modes of the SAC to kinetochore connection are present within plant lineages. These findings elucidate the conserved role of the KMN network in cell division and a striking dynamic of evolutionary patterns in the SAC signaling and kinetochore network.

Kinetochore assembly throughout the cell cycle

The kinetochore plays an essential role in facilitating chromosome segregation during cell division. This massive protein complex assembles onto the centromere of chromosomes and enables their attachment to spindle microtubules during mitosis. The kinetochore also functions as a signaling hub to regulate cell cycle progression, and is crucial to ensuring the fidelity of chromosome segregation. Despite the fact that kinetochores are large and robust molecular assemblies, they are also highly dynamic structures that undergo structural and organizational changes throughout the cell cycle. This review will highlight our current understanding of kinetochore structure and function, focusing on the dynamic processes that underlie kinetochore assembly.

From the Nuclear Pore to the Fibrous Corona: A MAD Journey to Preserve Genome Stability

The relationship between kinetochores and nuclear pore complexes (NPCs) is intimate but poorly understood. Several NPC components and associated proteins are relocated to mitotic kinetochores to assist in different activities that ensure faithful chromosome segregation. Such is the case of the Mad1-c-Mad2 complex, the catalytic core of the spindle assembly checkpoint (SAC), a surveillance pathway that delays anaphase until all kinetochores are attached to spindle microtubules. Mad1-c-Mad2 is recruited to discrete domains of unattached kinetochores from where it promotes the rate-limiting step in the assembly of anaphase-inhibitory complexes. SAC proficiency further requires Mad1-c-Mad2 to be anchored at NPCs during interphase. However, the mechanistic relevance of this arrangement for SAC function remains ill-defined. Recent studies uncover the molecular underpinnings that coordinate the release of Mad1-c-Mad2 from NPCs with its prompt recruitment to kinetochores. Here, current knowledge on Mad1-c-Mad2 function and spatiotemporal regulation is reviewed and the critical questions that remain unanswered are highlighted.

Dynamics of kinetochore structure and its regulations during mitotic progression

Faithful chromosome segregation during mitosis in eukaryotes requires attachment of the kinetochore, a large protein complex assembled on the centromere of each chromosome, to the spindle microtubules. The kinetochore is a structural interface for the microtubule attachment and provides molecular surveillance mechanisms that monitor and ensure the precise microtubule attachment as well, including error correction and spindle assembly checkpoint. During mitotic progression, the kinetochore undergoes dynamic morphological changes that are observable through electron microscopy as well as through fluorescence microscopy. These structural changes might be associated with the kinetochore function. In this review, we summarize how the dynamics of kinetochore morphology are associated with its functions and discuss recent findings on the switching of protein interaction networks in the kinetochore during cell cycle progression.

Mutations in the Drosophila rough deal gene affecting RZZ kinetochore function

BACKGROUND

The RZZ complex, composed of the proteins Rough-Deal (Rod), Zw10 and Zwilch, plays a central role in the spindle assembly checkpoint (SAC), which assures proper sister chromatid segregation during mitosis. RZZ contributes to the regulation of the spindle assembly checkpoint by helping to recruit Mad1-Mad2 and the microtubule motor dynein to unattached kinetochores. It is an important component of the outer kinetochore and specifically the fibrous corona whose expansion is believed to facilitate microtubule capture. How RZZ carries out its diverse activities is only poorly understood. The C-terminal region of the Rod subunit is relatively well-conserved across metazoan phylogeny, but no function has been attributed to it.

RESULTS

To explore the importance of the Rod_C domain in RZZ function in Drosophila, we generated a series of point mutations in a stretch of 200 residues within this domain and we report here their phenotypes. Several of the mutations profoundly disrupt recruitment of RZZ to kinetochores, including one in a temperature-sensitive manner, while still retaining the capacity to assemble into a complex with Zw10 and Zwilch. Others affect aspects of dynein activity or recruitment at the kinetochore.

CONCLUSIONS AND SIGNIFICANCE

These results suggest that the Rod_C domain participates in the protein interactions necessary for RZZ recruitment and functionality at kinetochores.

Cyclin B1 scaffolds MAD1 at the kinetochore corona to activate the mitotic checkpoint

Cyclin B:CDK1 is the master kinase regulator of mitosis. We show here that, in addition to its kinase functions, mammalian Cyclin B also scaffolds a localised signalling pathway to help preserve genome stability. Cyclin B1 localises to an expanded region of the outer kinetochore, known as the corona, where it scaffolds the spindle assembly checkpoint (SAC) machinery by binding directly to MAD1. In vitro reconstitutions map the key binding interface to a few acidic residues in the N-terminal region of MAD1, and point mutations in this sequence abolish MAD1 corona localisation and weaken the SAC. Therefore, Cyclin B1 is the long-sought-after scaffold that links MAD1 to the corona, and this specific pool of MAD1 is needed to generate a robust SAC response. Robustness arises because Cyclin B1:MAD1 localisation loses dependence on MPS1 kinase after the corona has been established, ensuring that corona-localised MAD1 can still be phosphorylated when MPS1 activity is low. Therefore, this study explains how corona-MAD1 generates a robust SAC signal, and it reveals a scaffolding role for the key mitotic kinase, Cyclin B1:CDK1, which ultimately helps to inhibit its own degradation.

Crowning the Kinetochore: The Fibrous Corona in Chromosome Segregation

The kinetochore is at the heart of chromosome segregation in mitosis and meiosis. Rather than a static linker complex for chromatin and spindle microtubules, it is highly dynamic in composition, size, and shape. While known for decades that it can expand and grow a fibrous meshwork known as the corona, it was until recently unclear what constitutes this 'crown' and what its relevance is for kinetochore function. Here, we highlight recent discoveries in fibrous corona biology, and place them in the context of the processes that orchestrate high-fidelity chromosome segregation.

Ensemble-Level Organization of Human Kinetochores and Evidence for Distinct Tension and Attachment Sensors

Kinetochores are multi-protein machines that form dynamic attachments to microtubules and control chromosome segregation. High fidelity is ensured because kinetochores can monitor attachment status and tension, using this information to activate checkpoints and error-correction mechanisms. To explore how kinetochores achieve this, we used two- and three-color subpixel fluorescence localization to define how proteins from six major complexes (CCAN, MIS12, NDC80, KNL1, RZZ, and SKA) and the checkpoint proteins Bub1, Mad1, and Mad2 are organized in the human kinetochore. This reveals how the outer kinetochore has a high nematic order and is largely invariant to the loss of attachment or tension, except for two mechanical sensors. First, Knl1 unravels to relay tension, and second, NDC80 undergoes jackknifing and loss of nematic order under microtubule detachment, with only the latter wired up to the checkpoint signaling system. This provides insight into how kinetochores integrate mechanical signals to promote error-free chromosome segregation.

The potential drug for treatment in pancreatic adenocarcinoma: a bioinformatical study based on distinct drug databases

Background

The prediction of drug-target interaction from chemical and biological data can advance our search for potential drug, contributing to a therapeutic strategy for pancreatic adenocarcinoma (PAAD). We aim to identify hub genes of PAAD and search for potential drugs from distinct databases. The docking simulation is adopted to validate our findings from computable perspective.

Methods

Differently expressed genes (DEGs) of PAAD were performed based on TCGA. With two Cytoscape plugins of CentiScaPe and MCODE, hub genes were analyzed and visualized by STRING analysis of Protein–protein Interaction (PPI). The hub genes were further selected with significant prognostic values. In addition, we examined the correlation between hub genes and immune infiltration in PAAD. Subsequently, we searched for the hub gene-targeted drugs in Connectivity map (Cmap) and cBioportal, which provided a large body of candidate drugs. The hub gene, which was covered in the above two databases, was estimated in Traditional Chinese Medicine Systems Pharmacology (TCMSP) and Herbal Ingredients’ Targets (HIT) database, which collected natural herbs and related ingredients. After obtaining molecular structures, the potential ingredient from TCMSP was applied for a docking simulation. We finalized a network connectivity of ingredient and its targets.

Results

A total of 2616 DEGs of PAAD were identified, then we further determined and visualized 24 hub genes by a connectivity analysis of PPI. Based on prognostic value, we identified 5 hub genes including AURKA (p = 0.0059), CCNA2 (p = 0.0047), CXCL10 (p = 0.0044), ADAM10 (p = 0.00043), and BUB1 (p = 0.0033). We then estimated tumor immune correlation of these 5 hub genes, because the immune effector process was one major result of GO analysis. Subsequently, we continued to search for candidate drugs from Cmap and cBioportal database. BUB1, not covered in the above two databases, was estimated in TCMSP and HIT databases. Our results revealed that genistein was a potential drug of BUB1. Next, we generated two docking modes to validate drug-target interaction based on their 3D structures. We eventually constructed a network connectivity of BUB1 and its targets.

Conclusions

All 5 hub genes that predicted poor prognosis had their potential drugs, especially our findings showed that genistein was predicted to target BUB1 based on TCMSP and docking simulation. This study provided a reasonable approach to extensively retrieve and initially validate putative therapeutic agents for PAAD. In future, these drug-target results should be investigated with solid data from practical experiments.

Effect of KNL1 on the proliferation and apoptosis of colorectal cancer cells

Objective:

To identify the expression of kinetochore scaffold 1 (KNL1) in colorectal tumor tissues and to clarify the role of this gene in the proliferation capability of colorectal cancer cells.

Methods:

A total of 108 paired colorectal tumor and normal tissue samples were collected from patients with colorectal cancer and subjected to quantitative polymerase chain reaction and immunohistochemistry analyses. Expression levels of KNL1 mRNA and protein were compared between tumor and normal tissues, and KNL1 levels were evaluated in relation to the patients’ tumor differentiation, sex, lymph node metastasis, TNM stage, infiltration depth, age, and tumor location. Survival curves were also constructed and compared between patients with tumor samples with and without KLN1 protein expression. KNL1 was under-expressed in colorectal cancer cells in vitro using lentiviral transfection with short hairpin RNA, and its function was evaluated by proliferation, colony-formation, and apoptosis assays. Expression levels of BUB1 protein were also compared between tumor and normal tissues, and the correlation between KNL1 expression and BUB1 expression in colorectal cancer tissues was examined.

Results:

KNL1 mRNA and protein were both highly expressed in colorectal tumor tissues compared with paired normal tissues. KNL1 downregulation significantly inhibited colorectal cancer cell proliferation and colony formation, and promoted apoptosis. KNL1 protein expression was significantly associated with tumor differentiation, but not with sex, lymph node metastasis, TNM stage, infiltration depth, age, or tumor location. KNL1 protein expression was also significantly associated with poorer survival. Moreover, there was a significant correlation between KNL1 and BUB1 in colorectal cancer tissues.

Conclusions:

KNL1 plays an effective role in decreasing apoptosis and promoting the proliferation of colorectal cancer cells, suggesting that its inhibition may represent a promising therapeutic approach in patients with colorectal cancer.

ULK1 phosphorylates Mad1 to regulate spindle assembly checkpoint

The spindle assembly checkpoint (SAC) ensures the fidelity of chromosome segregation during mitosis. Here, we show that ULK1, a serine/threonine kinase that plays a key role in initiation of autophagy, also has an important function in the activation of SAC. ULK1 phosphorylates the SAC protein Mad1 at Ser546 to recruit Mad1 to kinetochores. Furthermore, Rod/ZW10/Zwilch (RZZ) complex may serve as a receptor for phos-Ser546-Mad1 at kinetochore, since phosphorylation of Mad1 by ULK1 strengthens the interaction between Mad1 and RZZ complex. In addition, deletion of ULK1 increases chromosome instability and cytotoxicity of paclitaxel, resulting in significant impairment of cancer cell growth. These findings highlight the role of ULK1 as a protein kinase controlling the fidelity of chromosome segregation and cell-cycle progression.

Orchestration of the spindle assembly checkpoint by CDK1-cyclin B1

In mitosis, the spindle assembly checkpoint (SAC) monitors the formation of microtubule-kinetochore attachments during capture of chromosomes by the mitotic spindle. Spindle assembly is complete once there are no longer any unattached kinetochores. Here, we will discuss the mechanism and key components of spindle checkpoint signalling. Unattached kinetochores bind the principal spindle checkpoint kinase monopolar spindle 1 (MPS1). MPS1 triggers the recruitment of other spindle checkpoint proteins and the formation of a soluble inhibitor of anaphase, thus preventing exit from mitosis. On microtubule attachment, kinetochores become checkpoint silent due to the actions of PP2A-B56 and PP1. This SAC responsive period has to be coordinated with mitotic spindle formation to ensure timely mitotic exit and accurate chromosome segregation. We focus on the molecular mechanisms by which the SAC permissive state is created, describing a central role for CDK1-cyclin B1 and its counteracting phosphatase PP2A-B55. Furthermore, we discuss how CDK1-cyclin B1, through its interaction with MAD1, acts as an integral component of the SAC, and actively orchestrates checkpoint signalling and thus contributes to the faithful execution of mitosis.

Spindle checkpoint silencing at kinetochores with submaximal microtubule occupancy

The spindle assembly checkpoint (SAC) ensures proper chromosome segregation by monitoring kinetochore-microtubule interactions. SAC proteins are shed from kinetochores once stable attachments are achieved. Human kinetochores consist of hundreds of SAC protein recruitment modules and bind up to 20 microtubules, raising the question of how the SAC responds to intermediate attachment states. We show that one protein module ('RZZS-MAD1-MAD2') of the SAC is removed from kinetochores at low microtubule occupancy and remains absent at higher occupancies, while another module ('BUB1-BUBR1') is retained at substantial levels irrespective of attachment states. These behaviours reflect different silencing mechanisms: while BUB1 displacement is almost fully dependent on MPS1 inactivation, MAD1 (also known as MAD1L1) displacement is not. Artificially tuning the affinity of kinetochores for microtubules further shows that ∼50% occupancy is sufficient to shed MAD2 and silence the SAC. Kinetochores thus respond as a single unit to shut down SAC signalling at submaximal occupancy states, but retain one SAC module. This may ensure continued SAC silencing on kinetochores with fluctuating occupancy states while maintaining the ability for fast SAC re-activation.

Robust elimination of genome-damaged cells safeguards against brain somatic aneuploidy following Knl1 deletion

The brain is a genomic mosaic shaped by cellular responses to genome damage. Here, we manipulate somatic genome stability by conditional Knl1 deletion from embryonic mouse brain. KNL1 mutations cause microcephaly and KNL1 mediates the spindle assembly checkpoint, a safeguard against chromosome missegregation and aneuploidy. We find that following Knl1 deletion, segregation errors in mitotic neural progenitor cells give rise to DNA damage on the missegregated chromosomes. This triggers rapid p53 activation and robust apoptotic and microglial phagocytic responses that extensively eliminate cells with somatic genome damage, thus causing microcephaly. By leaving only karyotypically normal progenitors to continue dividing, these mechanisms provide a second safeguard against brain somatic aneuploidy. Without Knl1 or p53-dependent safeguards, genome-damaged cells are not cleared, alleviating microcephaly, but paradoxically leading to total pre-weaning lethality. Thus, mitotic genome damage activates robust responses to eliminate somatic mutant cells, which if left unpurged, can impact brain and organismal fitness.

BUB1 Is Essential for the Viability of Human Cells in which the Spindle Assembly Checkpoint Is Compromised

The spindle assembly checkpoint (SAC) ensures faithful segregation of chromosomes. Although most mammalian cell types depend on the SAC for viability, we found that human HAP1 cells can grow SAC independently. We generated MAD1- and MAD2-deficient cells and mutagenized them to identify synthetic lethal interactions, revealing that chromosome congression factors become essential upon SAC deficiency. Besides expected hits, we also found that BUB1 becomes essential in SAC-deficient cells. We found that the BUB1 C terminus regulates alignment as well as recruitment of CENPF. Second, we found that BUBR1 was not essential in SAC-deficient HAP1 cells. We confirmed that BUBR1 does not regulate chromosome alignment in HAP1 cells and that BUB1 does not regulate chromosome alignment through BUBR1. Taken together, our data resolve some long-standing questions about the interplay between BUB1 and BUBR1 and their respective roles in the SAC and chromosome alignment.

Recent Progress on the Localization of the Spindle Assembly Checkpoint Machinery to Kinetochores

Faithful chromosome segregation during mitosis is crucial for maintaining genome stability. The spindle assembly checkpoint (SAC) is a surveillance mechanism that ensures accurate mitotic progression. Defective SAC signaling leads to premature sister chromatid separation and aneuploid daughter cells. Mechanistically, the SAC couples the kinetochore microtubule attachment status to the cell cycle progression machinery. In the presence of abnormal kinetochore microtubule attachments, the SAC prevents the metaphase-to-anaphase transition through a complex kinase-phosphatase signaling cascade which results in the correct balance of SAC components recruited to the kinetochore. The correct kinetochore localization of SAC proteins is a prerequisite for robust SAC signaling and, hence, accurate chromosome segregation. Here, we review recent progresses on the kinetochore recruitment of core SAC factors.

Efficient mitotic checkpoint signaling depends on integrated activities of Bub1 and the RZZ complex

Kinetochore localized Mad1 is essential for generating a "wait anaphase" signal during mitosis, hereby ensuring accurate chromosome segregation. Inconsistent models for the function and quantitative contribution of the two mammalian Mad1 kinetochore receptors: Bub1 and the Rod-Zw10-Zwilch (RZZ) complex exist. By combining genome editing and RNAi, we achieve penetrant removal of Bub1 and Rod in human cells, which reveals that efficient checkpoint signaling depends on the integrated activities of these proteins. Rod removal reduces the proximity of Bub1 and Mad1, and we can bypass the requirement for Rod by tethering Mad1 to kinetochores or increasing the strength of the Bub1-Mad1 interaction. We find that Bub1 has checkpoint functions independent of Mad1 localization that are supported by low levels of Bub1 suggesting a catalytic function. In conclusion, our results support an integrated model for the Mad1 receptors in which the primary role of RZZ is to localize Mad1 at kinetochores to generate the Mad1-Bub1 complex.

Ectopic Activation of the Spindle Assembly Checkpoint Signaling Cascade Reveals Its Biochemical Design

Switch-like activation of the spindle assembly checkpoint (SAC) is critical for accurate chromosome segregation and for cell division in a timely manner. To determine the mechanisms that achieve this, we engineered an ectopic, kinetochore-independent SAC activator: the "eSAC." The eSAC stimulates SAC signaling by artificially dimerizing Mps1 kinase domain and a cytosolic KNL1 phosphodomain, the kinetochore signaling scaffold. By exploiting variable eSAC expression in a cell population, we defined the dependence of the eSAC-induced mitotic delay on eSAC concentration in a cell to reveal the dose-response behavior of the core signaling cascade of the SAC. These quantitative analyses and subsequent mathematical modeling of the dose-response data uncover two crucial properties of the core SAC signaling cascade: (1) a cellular limit on the maximum anaphase-inhibitory signal that the cascade can generate due to the limited supply of SAC proteins and (2) the ability of the KNL1 phosphodomain to produce the anaphase-inhibitory signal synergistically, when it recruits multiple SAC proteins simultaneously. We propose that these properties together achieve inverse, non-linear scaling between the signal output per kinetochore and the number of signaling kinetochores. When the number of kinetochores is low, synergistic signaling by KNL1 enables each kinetochore to produce a disproportionately strong signal output. However, when many kinetochores signal concurrently, they compete for a limited supply of SAC proteins. This frustrates synergistic signaling and lowers their signal output. Thus, the signaling activity of unattached kinetochores will adapt to the changing number of signaling kinetochores to enable the SAC to approximate switch-like behavior.

BUB-1 promotes amphitelic chromosome biorientation via multiple activities at the kinetochore

Accurate chromosome segregation relies on bioriented amphitelic attachments of chromosomes to microtubules of the mitotic spindle, in which sister chromatids are connected to opposite spindle poles. BUB-1 is a protein of the Spindle Assembly Checkpoint (SAC) that coordinates chromosome attachment with anaphase onset. BUB-1 is also required for accurate sister chromatid segregation independently of its SAC function, but the underlying mechanism remains unclear. Here we show that, in Caenorhabditis elegans embryos, BUB-1 accelerates the establishment of non-merotelic end-on kinetochore-microtubule attachments by recruiting the RZZ complex and its downstream partner dynein-dynactin at the kinetochore. In parallel, BUB-1 limits attachment maturation by the SKA complex. This activity opposes kinetochore-microtubule attachment stabilisation promoted by CLS-2^CLASP-dependent kinetochore-microtubule assembly. BUB-1 is therefore a SAC component that coordinates the function of multiple downstream kinetochore-associated proteins to ensure accurate chromosome segregation.

Cell Division: The Unattached Kinetochore Wears an Expansive RZZ Coat

The Rod-Zw10-Zwilch complex localizes to kinetochores during mitosis. New studies reveal that this complex plays a critical role in driving the expansion of the outer domain of unattached kinetochores, in addition to its known role in the control of the spindle assembly checkpoint.

Self-Assembly of the RZZ Complex into Filaments Drives Kinetochore Expansion in the Absence of Microtubule Attachment

The kinetochore is a dynamic multi-protein assembly that forms on each sister chromatid and interacts with microtubules of the mitotic spindle to drive chromosome segregation. In animals, kinetochores without attached microtubules expand their outermost layer into crescent and ring shapes to promote microtubule capture and spindle assembly checkpoint (SAC) signaling. Kinetochore expansion is an example of protein co-polymerization, but the mechanism is not understood. Here, we present evidence that kinetochore expansion is driven by oligomerization of the Rod-Zw10-Zwilch (RZZ) complex, an outer kinetochore component that recruits the motor dynein and the SAC proteins Mad1-Mad2. Depletion of ROD in human cells suppresses kinetochore expansion, as does depletion of Spindly, the adaptor that connects RZZ to dynein, although dynein itself is dispensable. Expansion is also suppressed by mutating ZWILCH residues implicated in Spindly binding. Conversely, supplying cells with excess ROD facilitates kinetochore expansion under otherwise prohibitive conditions. Using the C. elegans early embryo, we demonstrate that ROD-1 has a concentration-dependent propensity for oligomerizing into micrometer-scale filaments, and we identify the ROD-1 β-propeller as a key regulator of self-assembly. Finally, we show that a minimal ROD-1-Zw10 complex efficiently oligomerizes into filaments in vitro. Our results suggest that RZZ's capacity for oligomerization is harnessed by kinetochores to assemble the expanded outermost domain, in which RZZ filaments serve as recruitment platforms for SAC components and microtubule-binding proteins. Thus, we propose that reversible RZZ self-assembly into filaments underlies the adaptive change in kinetochore size that contributes to chromosome segregation fidelity.

Distinct Roles of RZZ and Bub1-KNL1 in Mitotic Checkpoint Signaling and Kinetochore Expansion

The Mad1-Mad2 heterodimer is the catalytic hub of the spindle assembly checkpoint (SAC), which controls M phase progression through a multi-subunit anaphase inhibitor, the mitotic checkpoint complex (MCC) [1, 2]. During interphase, Mad1-Mad2 generates MCC at nuclear pores [3]. After nuclear envelope breakdown (NEBD), kinetochore-associated Mad1-Mad2 catalyzes MCC assembly until all chromosomes achieve bipolar attachment [1, 2]. Mad1-Mad2 and other factors are also incorporated into the fibrous corona, a phospho-dependent expansion of the outer kinetochore that precedes microtubule attachment [4-6]. The factor(s) involved in targeting Mad1-Mad2 to kinetochores in higher eukaryotes remain controversial [7-12], and the specific phosphorylation event(s) that trigger corona formation remain elusive [5, 13]. We used genome editing to eliminate Bub1, KNL1, and the Rod-Zw10-Zwilch (RZZ) complex in human cells. We show that RZZ's sole role in SAC activation is to tether Mad1-Mad2 to kinetochores. Separately, Mps1 kinase triggers fibrous corona formation by phosphorylating two N-terminal sites on Rod. In contrast, Bub1 and KNL1 activate kinetochore-bound Mad1-Mad2 to produce a "wait anaphase" signal but are not required for corona formation. We also show that clonal lines isolated after BUB1 disruption recover Bub1 expression and SAC function through nonsense-associated alternative splicing (NAS). Our study reveals a fundamental division of labor in the mammalian SAC and highlights a transcriptional response to nonsense mutations that can reduce or eliminate penetrance in genome editing experiments.

CHMP4C: A novel regulator of the mitotic spindle checkpoint

The mitotic spindle checkpoint delays anaphase onset until all chromosomes have achieved stable kinetochore-microtubule attachments. Here, we discuss recent findings showing that CHMP4C, a component of the endosomal sorting complex required for transport (ESCRT) machinery, protects human cells against chromosome missegregation by promoting localisation of the ROD-ZW10-ZWILCH (RZZ) spindle checkpoint complex to unattached kinetochores.

Dynamic regulation of dynein localization revealed by small molecule inhibitors of ubiquitination enzymes

Cytoplasmic dynein is a minus-end-directed microtubule-based motor that acts at diverse subcellular sites. During mitosis, dynein localizes simultaneously to the mitotic spindle, spindle poles, kinetochores and the cell cortex. However, it is unclear what controls the relative targeting of dynein to these locations. As dynein is heavily post-translationally modified, we sought to test a role for these modifications in regulating dynein localization. We find that dynein rapidly and strongly accumulates at mitotic spindle poles following treatment with NSC697923, a small molecule that inhibits the ubiquitin E2 enzyme, Ubc13, or treatment with PYR-41, a ubiquitin E1 inhibitor. Subsets of dynein regulators such as Lis1, ZW10 and Spindly accumulate at the spindle poles, whereas others do not, suggesting that NSC697923 differentially affects specific dynein populations. We additionally find that dynein relocalization induced by NSC697923 or PYR-41 can be suppressed by simultaneous treatment with the non-selective deubiquitinase inhibitor, PR-619. However, we did not observe altered dynein localization following treatment with the selective E1 inhibitor, TAK-243. Although it is possible that off-target effects of NSC697923 and PYR-41 are responsible for the observed changes in dynein localization, the rapid relocalization upon drug treatment highlights the highly dynamic nature of dynein regulation during mitosis.